Updating cookie values - php

Question

I have a shopping cart using cookies, you can add products to it by going to the product-detail page and click on add cart. The script needs to add +1 to the quantity in the shopping cart, everytime you can click on add. I don't know why, but the quantity stays 1 everytime.
Basically, the question is: Why doesn't it update the quantity within the cookie?
Product.php:
if(isset($_POST['add'])){
if(!empty($_POST['m'])){
if (isset($_COOKIE['cart'])){
$cart = json_decode($_COOKIE['cart'], TRUE, 512, JSON_OBJECT_AS_ARRAY); // if cookie is set, get the contents of it
} else{
$cart = [];// else create an empty cart
}
// append new product and add to cart
$cart[$product['id']] = [];
$cart[$product['id']]['m'] = empty($_POST['m']) ? 1 : $_POST['m'];
if(!empty($cart[$product['id']]['quantity'])){
$cart[$product['id']]['quantity'] += 1;
} else {
$cart[$product['id']]['quantity'] = 1;
}
setcookie('cart', json_encode($cart), time()+3600, '/');
} else {
$error = "U moet minimaal 1m invullen";
}
}
Also in the shopping cart itself, i need to be able to modify the quantity, this value is allowed to be overwritten.
shoppingcart.php:
if(isset($_COOKIE['cart'])){
$cart = json_decode($_COOKIE['cart'], TRUE, 512, JSON_OBJECT_AS_ARRAY);
} else {
$cart = [];
}
// dd($cart);
if(isset($_POST['remove'])){
unset($cart[$_POST['item']]);
setcookie('cart', json_encode($cart), time()+3600, '/');
}
$list = $model->selectMultipleById($cart, 'carpet');

Answer 1

Try this:
// append new product and add to cart
//first test if you have the id in your cookie. if so: update qty +1
if(!empty($cart[$product['id']])){
$cart[$product['id']]['quantity'] += 1;
}
//else create a new item in the cookie
else {
$cart[$product['id']] = [];
$cart[$product['id']]['quantity'] = 1;
}
//now $cart[$product['id']]['m'] always exists, so you can update m
$cart[$product['id']]['m'] = empty($_POST['m']) ? 1 : $_POST['m'];

Answer 2

This is what i tried but i seem to repeat myself with my code.
if(isset($_COOKIE['cart'][$product['id']])){
$cart[$product['id']]['m'] = empty($_POST['m']) ? 1 : $_POST['m'];
if(!empty($cart[$product['id']]['quantity'])){
$cart[$product['id']]['quantity'] += 1;
} else {
$cart[$product['id']]['quantity'] = 1;
}
} else {
$cart[$product['id']] = [];
$cart[$product['id']]['m'] = empty($_POST['m']) ? 1 : $_POST['m'];
$cart[$product['id']]['quantity'] = 1;
}